Cryogenic air separation unit with flexible liquid product make

ABSTRACT

A cryogenic air separation unit that provides flexibility in the production of liquid products is disclosed. The present cryogenic air separation unit and associated operating methods involves the use of a dual nozzle arrangement for the main heat exchanger that allows a turbine air stream draw from the main heat exchanger at different temperatures to provide refrigeration to the cryogenic air separation unit which, in turn, enables different production modes for the various liquid products.

TECHNICAL FIELD

The present invention relates to a cryogenic air separation unit, andmore particularly, to a system and method for providing flexibility inthe production of liquid products from the cryogenic air separationunit. Still more particularly, the present system and method involves adual nozzle arrangement for a main heat exchanger of a cryogenic airseparation unit that allows a turbine air stream draw from the main heatexchanger at one of two different temperatures to provide supplementalrefrigeration required for different liquid product makes. For sake ofclarity, a cryogenic air separation unit as used herein refers to asystem and apparatus for separation of air into by its components,namely oxygen, nitrogen, and argon by means of the fractionaldistillation process.

BACKGROUND

It has long been known to separate air by cryogenic rectification, ormore specifically a fractional distillation process. In such processes,the incoming feed air to be separated is pressurized, purified and thencooled to a temperature suitable for its rectification and thenintroduced into one or more distillation columns. Each of thedistillation columns has various mass transfer elements such as trays orpacking, for example, structured packing, which bring liquid and vaporphases of the gaseous mixtures within the distillation columns intocontact with one another and effectuate mass transfer between the vaporand liquid phases. The incoming feed air stream is thereby distilledwithin the distillation column or columns to form component streamsenriched in the components of the gaseous mixture, namely nitrogen,oxygen or argon. The nitrogen, oxygen and argon streams can be taken asliquid products and/or gaseous products and are typically used in thecooling of the incoming feed air, which takes place through indirectheat exchange within a main heat exchanger. For example, it is wellknown to mechanically pump a liquid product, such as an oxygen-richliquid column bottoms stream to the main heat exchanger where it isvaporized against the liquefying compressed air stream.

Additional or supplemental refrigeration for the cryogenic airseparation unit is often generated by expanding a stream made up of aportion of the compressed and purified feed air in a turboexpander andintroducing the expanded stream into at least one of the distillationcolumns. The additional refrigeration provided by the expansion of theportion of the compressed and purified feed air in the turboexpandertypically offsets any refrigeration losses in the warm end of thecryogenic air separation unit and/or enables production of liquidproducts from the cryogenic air separation unit.

Most cryogenic air separation units are typically designed, constructedand operated to meet the base load product slate requirements for one ormore end-user customers and optionally the local or merchant liquidproduct market demand. The base load product slate requirementstypically include a target volume of high pressure gaseous oxygen, aswell as various co-products such as gaseous nitrogen, liquid oxygen,liquid nitrogen, and/or liquid argon. The air separation units aredesigned and operated based, in part, on the selected design conditions,including the target product slate, typical day ambient conditions aswell as the available utility/power supply costs and conditions.

Over time, the product slate requirements for some air separation unitschange, and in particular, there are often changes in local demand forliquid products from merchant customers in the area surrounding the airseparation units. Other changes to product slate requirements may arisein a shorter timeframe when, for example, the production of highervolumes of liquid products (i.e. higher liquid make) is moreeconomically feasible due to lower utility/power costs.

To meet these varying liquid product demands, typically expressed as apercent of the incoming feed air, it is desirable to change the airseparation unit operating characteristics in order to adjust the productslate requirements. It is well known that the liquid production raterequires supplemental refrigeration and that the additional orsupplemental refrigeration provided by the expansion of the compressedand purified feed air in the turboexpander is dependent on thetemperature and flow of the compressed air stream directed to theturboexpander, as well as the pressure ratio across the turboexpander.Thus, in order to change liquid production rates, it is conventionalpractice to adjust the flow of the compressed air stream to theturboexpander. See, for example, U.S. Pat. No. 5,412,953.

Another possibility in controlling liquid production from a cryogenicair separation unit is to vary the expansion ratio of the turboexpanderby increasing or decreasing the pressure of the compressed air stream tothe turboexpander. This prior art solution can result in a poor turbineefficiency and control problems in that as the pressure changes at afixed temperature, the volume flow of the turbine air often changessignificantly and gets far away from the best efficiency condition. Atthe extreme condition, the turbine air stream to be expanded may even beliquefied at the exhaust or outlet of the turboexpander. In suchsituations, the turboexpander would not only suffer from poorefficiency, but also may incur potential damage as a result of suchunintended liquefaction. At the other extreme, as the pressure of theturbine air stream is decreased at a fixed inlet temperature, thetemperature of the expanded stream increases. If the exhaust streamtemperature is above the saturation temperature of the stream feed tothe distillation column, liquids within the distillation column mayvaporize resulting in high local vapor flows, loss of separationperformance and potential distillation column flooding.

It has been known to control the turboexpander inlet temperature of anair separation unit in order to prevent liquefaction in theturboexpander exhaust. One such example of controlling turbine airstream temperature is disclosed in U.S. Pat. No. 3,355,901. In thisprior art system the turbine air stream is comprised of a mixture of twostreams, the first stream being a compressed and purified stream of airthat is cooled in the main heat exchanger and the second stream being acompressed and purified stream of air that bypasses the heat exchanger.The first and second streams are then combined and introduced into theinlet of the turboexpander. By controlling the relative flows of the twostreams, the temperature of the turbine air stream is controlled. Morespecifically, the disclosed control system senses the turboexpanderexhaust temperature which is fed as an input into the control system tocontrol a valve that in turn controls flow of one of the two streams,namely the first stream that is cooled within the main heat exchanger.

Another example of controlling the turbine air stream temperature isdisclosed in U.S. Pat. Nos. 8,020,408 and 9,038,413. In these prior artsystems the turbine air stream is also comprised of a mixture of twostreams. The first stream is a compressed and purified stream of airthat is partially cooled in the main heat exchanger to a firsttemperature and the second stream being is a compressed and purifiedstream of air that is partially cooled in the main heat exchanger to asecond temperature. The first and second streams are then combined in adownstream static mixer before being introduced into the inlet of theturboexpander. The flow rates of the two streams from the dual nozzlesof the main heat exchanger are adjusted to control the inlet temperatureto the turboexpander supplying refrigeration and to minimize potentialdeviation of the turboexpander exhaust from a saturated vapor state.

The prior art dual nozzle arrangements disclosed in U.S. Pat. Nos.9,038,413 and 8,020,408 have several disadvantages, specifically alikely higher pressure drop resulting from the split paths and multipleopen or partially open valve arrangements required by the disclosedsystem and potential entropy loss that would likely occur when thecolder second stream is mixed with warmer first stream.

What is needed therefore, is an improved dual nozzle arrangement of amain heat exchanger of an air separation unit that allows for changes oradjustments in liquid production rates from the air separation unit bymeans of adjusting the inlet temperature of the compressed turbine airstream to the turboexpander in either a very short timeframe rapidchanges to liquid product make due to utility/power costs) or in longertimeframes (i.e. longer term changes in product slate requirements fromlocal merchant customers). Preferably, the improved dual nozzlearrangement of the main heat exchanger of an air separation unit allowswithdrawal of a turbine air stream from the main heat exchanger at oneof two different temperatures to provide the supplemental refrigerationrequired for different liquid product makes.

SUMMARY OF THE INVENTION

The present invention may be characterized as an air separation unitcomprising: (i) a main compressor arrangement configured to compress thefeed air stream; (ii) an adsorption based pre-purifier configured toproduce a compressed and purified feed air stream; (iii) a feed airstream circuit configured to divide the compressed and purified feed airstream into at least two streams including a boiler air stream and aturbine air stream; wherein the boiler air stream is further compressedand cooled in a main heat exchanger to a cold-end temperature; (iv) theturbine air stream is optionally compressed and also partially⁻ cooledin the main heat exchanger, wherein the partially cooled turbine airstream is discharged from the main heat exchanger as a first subsidiarystream at a first temperature wanner than the cold-end temperature or asa second subsidiary stream at a second temperature colder than the firsttemperature but warmer than the cold-end temperature. The partiallycooled first subsidiary stream or the partially cooled second subsidiarystream is then expanded to produce an exhaust stream which is introducedinto the distillation column system of the air separation unit toproduce products, including one or more liquid products.

The air separation unit is further configured to operate in a low liquidmake mode with the turboexpander configured to expand only the firstsubsidiary stream or to operate in a high liquid make mode with theturboexpander configured to expand only the second subsidiary stream.The low liquid make mode produces the liquid products in the amount ofabout 2.5 percent or less of the total feed air stream whereas the highliquid make mode produces the liquid products in the amount of about 4.5percent or more of the total feed air stream. Some embodiments of theair separation unit can also operate in a medium liquid make variablemode with the turboexpander configured to expand a mixture of the firstsubsidiary stream and the second subsidiary stream. Such medium liquidmake variable mode producing the liquid products in the amount ofbetween about 2 percent and about 5 percent of the total feed airstream.

The turbine air stream preferably comprises between about 60 percent and75 percent of the compressed and purified feed air stream. In someembodiments of the present air separation unit, a portion of the turbineair stream is fully cooled within the main heat exchanger to thecold-end temperature and at a medium pressure greater than the pressureof the compressed and purified feed air stream and less than thepressure of the compressed boiler air stream. This third subsidiarymedium pressure stream preferably bypasses the turboexpander and ispassed directly to the distillation columns.

The different operating modes of the cryogenic air separation unit arepreferably enabled by controlling a first valve disposed between themain heat exchanger and the turboexpander that is configured to adjustthe flow of the first subsidiary stream a second valve disposed betweenthe main heat exchanger and the turboexpander and configured to adjustthe flow of the second subsidiary stream. Adjusting the flow of thefirst subsidiary stream and second subsidiary stream exiting the mainheat exchanger adjusts the inlet temperature of the stream sent to theturboexpander and the corresponding outlet temperature of the exhauststream. The first valve may be an on-off valve configured to be in anopen position allowing flow of the first subsidiary stream to exit themain heat exchanger or a closed position preventing flow of the firstsubsidiary stream from exiting the main heat exchanger. Likewise, thesecond valve may also be an on-off valve configured to be in an openposition allowing flow of the second subsidiary stream to exit the mainheat exchanger or a closed position preventing flow of the secondsubsidiary stream from exiting the main heat exchanger.

In yet other embodiments of the present cryogenic air separation unit,there is included a bypass circuit configured to direct all or a portionof the turbine air stream to bypass the one or more turbine air streamcompressors. Where the bypass circuit arrangement is used the airseparation unit is configured to operate in one or more medium liquidmake modes with all or a portion of the turbine air stream bypassing atleast one of the one or more compressors prior to entering the main heatexchanger. The bypassed turbine air stream is then partially cooled inthe main heat exchanger and discharged therefrom as the first subsidiarystream at a first temperature warmer than the cold-end temperature or asthe second subsidiary stream at a second temperature colder than thefirst temperature but warmer than the cold-end temperature. Thepartially cooled first subsidiary stream or the partially cooled secondsubsidiary stream is then expanded to produce an exhaust stream which isintroduced into the distillation column system of the air separationunit to produce products, including one or more liquid products. In suchmedium liquid make modes, liquid products can be produced at a level ofbetween about 2 percent and about 5 percent of the total feed airstream.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention concludes with claims distinctly pointingout the subject matter that Applicants regard as their invention, it isbelieved that the invention will be better understood when taken inconnection with the accompanying drawings in which:

FIG. 1 is a schematic representation of a cryogenic air separation unitin accordance with the present invention;

FIG. 2 is a schematic representation of yet another embodiment of acryogenic air separation unit in accordance with the present invention;

FIG. 3 is a schematic representation of a cryogenic air separation unitin accordance with the present invention;

FIG. 4 is a schematic representation of a cryogenic air separation unitin accordance with the present invention; and

FIG. 5 is a perspective view of a heat exchanger distributor suitablefor use with embodiments of the heat exchanger used in the presentinvention.

DETAILED DESCRIPTION

The improved dual nozzle arrangement of a main heat exchanger in an airseparation unit provided herein allows for rapid changes or adjustmentsin liquid production rates in a very short timeframe resulting fromexternal operating conditions such as changes in utility/power costs. Inaddition, the design of the main heat exchanger with dual nozzles allowsthe performance of the main heat exchanger to be altered in longertimeframes, when for example changes in base load product slaterequirements occur. By designing a main heat exchanger that is capableof satisfying different product slate requirements, and particularlydifferent liquid make modes, the flexibility of the air separation unitis improved.

Turning now to FIGS. 1-4, there is shown simplified illustrations of acryogenic air separation unit 10. In a broad sense, the cryogenic airseparation unit 10 includes a main feed air compression train 20, aturbine air compression circuit 30, a boiler air compression circuit 40,a heat exchanger arrangement 50, a turbine based refrigeration circuit60 and a distillation column system 70. As used herein, the main feedair compression train 20, the turbine air compression circuit 30, andthe booster air compression circuit 40, collectively comprise a‘warm-end’ air compression circuit. Similarly, the heat exchangerarrangement 50, portions of the turbine based refrigeration circuit 60and portions of the distillation column system 70 are referred to as the‘cold-end’ equipment that are typically housed in one or more insulatedcold boxes.

WARM END AIR COMPRESSION CIRCUIT

In the main feed compression train 20 shown in FIGS. 1-4, the incomingfeed air 22 is drawn through an air suction filter house and compressedin a multi-stage, intercooled main air compressor arrangement 24 to apressure that can be between about 5 bar(a) and about 15 bar(a). Thismain air compressor arrangement 24 may include integrally gearedcompressor stages or a direct drive compressor stages, arranged inseries or in parallel. The compressed air stream exiting the main aircompressor arrangement 24 is typically fed to an aftercooler withintegral demister to remove the free moisture in the stream. The heat ofcompression from the final stages of compression for the main aircompressor arrangement 24 is typically removed in one or moreaftercooler(s) by cooling the compressed feed air with cooling towerwater. The condensate from this aftercooler as well as some of theintercoolers in the main air compression arrangement is preferably pipedto a condensate tank and used to supply water to other portions of theair separation unit.

The cool, dry compressed air feed 26 is then purified in apre-purification unit 28 to remove high boiling contaminants from thecool, dry compressed air feed 26. The pre-purification unit 28, as iswell known in the art, typically contains two beds of alumina and/ormolecular sieve operating in accordance with a temperature and/orpressure swing adsorption cycle in which moisture and other impurities,such as carbon dioxide, water vapor and hydrocarbons, are adsorbed.While one of the beds is used for pre-purification of the cool, drycompressed air feed 26 while the other bed is regenerated, preferablywith a portion of the waste nitrogen from the air separation unit. Thetwo beds switch service periodically. Particulates are removed from thecompressed, pre-purified feed air in a dust filter disposed downstreamof the pre-purification unit 28 to produce the compressed, purified feedair stream 29.

As described in more detail below, the compressed, purified feed airstream 29 is separated into oxygen-rich, nitrogen-rich, and argon-richfractions in a plurality of distillation columns including a higherpressure column 72, a lower pressure column 74, and optionally, an argoncolumn arrangement, which may include the illustrated superstaged argoncolumn 76, an argon condenser 78 which are coupled to the lower pressurecolumn 74 and arranged or configured with to produce a crude argonstream 170.

Prior to such distillation however, the compressed, pre-purified feedair stream 29 is split into a plurality of feed air streams, including aboiler air stream 42 and a turbine air stream 32 described in moredetail below. The boiler air stream 42 and turbine air stream 32 arefurther compressed and cooled to temperatures required for fractionaldistillation in the distillation columns. Cooling the boiler air stream42 to cryogenic temperatures of between about 96 Kelvin and 100 Kelvinis preferably accomplished by way of indirect heat exchange in a primaryor main heat exchanger 52 with the warming streams which include the oneor more oxygen, nitrogen and/or argon streams from the distillationcolumn system 70. Refrigeration for the cryogenic air separation unit 10is also typically generated by the turbine air stream 32 and associatedcold and/or warm turbine arrangements disposed within the turbine basedrefrigeration circuits 60 and/or any optional closed loop warmrefrigeration circuits.

In the illustrated embodiments, the compressed and purified feed airstream is divided into a first stream 42, and a second stream 32. Firststream 42, often referred to as the boiler air stream, is generallyabout 25% to 40% of the compressed and purified feed air stream and isyet further compressed within a boiler air stream compressor arrangement40, which preferably comprises yet another single or multi-stageintercooled compressor 41 and aftercooler 43. As with the main aircompressor arrangement 20, this boiler air stream compressor arrangement40 may include an integrally geared compressor or a direct drivecompressor. This boiler air stream compressor arrangement 40 furthercompresses the first boiler air stream 42 to a targeted pressure betweenabout 25 bar(a) and about 70 bar(a) to produce a further compressedboiler air stream 49. The further compressed boiler air stream 49 isdirected or introduced into aftercooler 43 to produce the compressedboiler air stream 45. Compressed boiler air stream 45 is then directedto main heat exchanger 52 where it is used to boil a liquid oxygenstream 188 via indirect heat exchange to produce a high pressure gaseousoxygen product stream 190. The compressed and cooled boiler air stream45 is may be subsequently divided into air streams 46A and 46B which arethen partially expanded in expansion valve(s) 47 and 48 and forintroduction into the lower pressure column 74 and higher pressurecolumn 72 respectively. The target pressure of the compressed boiler airstream 45 is generally dictated by the product requirements for the highpressure gaseous oxygen product stream 190. The temperature of thecooled and compressed boiler air stream 46 exiting the main heatexchanger 52 is preferably between about 96 Kelvin and 100 Kelvin whichrepresents a cold-end temperature of the main heat exchanger. In manyapplications, the boiler air compressor 41, turbine air compressor 34,turbine booster compressor 36 and turbine 62 could be configured in oneintegrally geared machine.

As illustrated, second stream 32, often referred to as the turbine airstream 32, is generally about 60% to 75% of the compressed and purifiedfeed air stream 29 and is optionally further compressed in one or moreturbine air compressors 34 and 36, cooled in aftercoolers 37 anddirected as stream 35 to the main heat exchanger 52 where it ispartially cooled prior to being directed to the turbine basedrefrigeration circuit 60, as described below. The target pressure of thefurther compressed turbine air stream 35 is preferably between about 20bar(a) and about 60 bar(a).

In some embodiments of the present system (See FIGS. 3 and 4), liquidproduction in the cryogenic air separation unit 1, including apressurized liquid oxygen product stream and a liquid nitrogen productstream, may be further varied by varying the pressure in the turbine airstream to the turboexpander. This variation in pressure can beeffectuated by a turbine air stream bypass circuit 39 which includes abypass line having a bypass valve 38 that can be set in an open orclosed position.

The bypass circuit is configured to direct all or a portion of theturbine air stream to bypass at least one of the one or more turbine aircompressors 36. In the illustrated embodiments, the bypassed turbine airstream 31 is diverted via bypass valve 38 directly to the main heatexchanger 52 or other heat exchanger used in the turbine basedrefrigeration circuit 60. Preferably, the target pressure of thebypassed turbine air stream 31 is between about 10 bar(a) and about 30bar(a). Other means for varying the pressure in the turbine air streaminclude use of variable inlet vanes on one or more of the turbine aircompressors to vary the pressure of turbine air stream sent to theturboexpander. Also, in some embodiments that utilize the bypasscircuit, it may be advantageous to provide a source of make-up nitrogenthat is directed to the turbine air stream compressors in lieu of theturbine air stream so as to not damage the turbine air compressor. Insuch arrangements, the compressed make-up nitrogen may be vented ordirected to another location of the cryogenic air separation unit.

COLD END SYSTEMS AND EQUIPMENT

The main heat exchanger 52 is preferably a brazed aluminum plate-fintype heat exchanger. Such heat exchangers are advantageous due to theircompact design, high heat transfer rates and their ability to processmultiple streams. They are manufactured as fully brazed and weldedpressure vessels. For small air separation units, a heat exchangercomprising a single core may be sufficient whereas for larger airseparation units handling higher flows, the main heat exchanger may beconstructed from several cores connected in parallel or series.

Turbine based refrigeration circuits 60 are often referred to as eithera lower column turbine (LCT) arrangement or an upper column turbine(UCT) arrangement which are used to provide refrigeration to atwo-column or three column fractional distillation system 70. In the LCTarrangement shown in FIGS. 1 and 2, the compressed turbine air stream 35is preferably at a pressure in the range from between about 20 bar(a) toabout 60 bar(a). The compressed turbine air stream 35 is directed orintroduced into main heat exchanger 52 where it is partially cooled to atemperature in a range of between about 130 Kelvin and about 220 Kelvinto form a partially cooled, compressed turbine air stream 61 that issubsequently introduced into a turboexpander 62 to produce an exhauststream 64 that is introduced into the higher pressure column 72 ofdistillation column system 70. In some embodiments, turboexpander 62 maybe coupled with one or more turbine air compressors 34 and/or 36 used tofurther compress the turbine air stream, either directly or byappropriate gearing.

In the illustrated embodiments of the present cryogenic air separationunit 10, the heat exchanger associated with the turbine basedrefrigeration circuit 60 includes a dual nozzle arrangement. In the dualnozzle arrangement, the heat exchanger 52 is configured to receive theturbine air stream 35 and discharge a first subsidiary stream 66 at afirst temperature wanner than the cold-end temperature and/or dischargea second subsidiary stream 68 at a second temperature colder than thefirst temperature but warmer than the cold-end temperature. The firstsubsidiary stream 66 of the compressed turbine air stream that exits theheat exchanger 52 at a first location is partially cooled only totemperatures in a range of between about 140 Kelvin and about 220 Kelvinwhereas the second subsidiary stream of the compressed turbine airstream that exits the heat exchanger at a second location is partiallycooled to a colder temperature than the first subsidiary stream,preferably to temperatures between about 130 Kelvin and about 140Kelvin.

The dual nozzle arrangement further includes a first valve 67 disposedbetween the heat exchanger 52 and the turboexpander 62 and configured toadjust the flow of the first subsidiary stream 66 which in turn controlsthe inlet temperature to the turboexpander 62 and the correspondingoutlet temperature. The dual nozzle arrangement also includes a secondvalve 69 that is disposed between the heat exchanger 52 and theturboexpander 62 and configured to adjust the flow of the secondsubsidiary stream 68 which thus provides a different inlet temperatureto the turboexpander 62 and different corresponding outlet temperature.In the embodiments of FIGS. 2 and 4, the dual nozzle arrangementincludes a third valve 63 disposed between the heat exchanger 52 and theturboexpander 62 and configured to adjust the mixing of the first andsecond subsidiary streams and therefore produce a combined stream 61that is directed to the turboexpander 62 that has a variable inlettemperature between that of the first subsidiary stream and the secondsubsidiary stream.

The first valve 67 and second valve 69 may be simple ‘on-off’ valvesconfigured in either an open position allowing flow of the subsidiarystream to exit the heat exchanger at that particular nozzle andassociated temperature or a closed position preventing flow of thesubsidiary stream from exiting that particular nozzle. Alternatively isother embodiments, the first and second valves may be flow controlvalves having an actuator and a controller that is configured tomodulate the flow over a range of different flow rates. Theturboexpander 62 is configured to expand the first subsidiary stream 66or the second subsidiary stream 68 or a combined stream 61 which isintroduced into the at least one distillation column, preferably thehigher pressure column 72, of the cryogenic air separation unit 10.

While the turbine based refrigeration circuit 60 illustrated in FIGS. 1and 2 is preferably shown as a lower column turbine (LCT) circuitarrangement where the expanded exhaust stream is fed to the higherpressure column 72 of the distillation column system 70, it iscontemplated that the turbine based refrigeration circuit alternativelymay be an upper column turbine (UCT) circuit where the turbine exhauststream is directed to the lower pressure column. Still further, theturbine based refrigeration circuit may be a combination of an LCTcircuit and UCT circuit and/or even other variants such as partial lowercolumn turbine (PLCT).

Although not shown, in the alternate embodiments that employ a UCTarrangement the turbine air stream is also partially cooled in the dualnozzle heat exchanger. The first subsidiary stream exits the heatexchanger at a first temperature warmer than the cold-end temperature ofthe heat exchanger. The second subsidiary stream exits the heatexchanger at a second temperature colder than the first temperature butalso warmer than the cold-end temperature. Similar to the LCT basedembodiments, the dual nozzle arrangement includes a first valveconfigured to adjust the flow of the first subsidiary stream and asecond valve configured to adjust the flow of the second subsidiarystream. The dual nozzle arrangement optionally includes a third flowcontrol valve disposed in operative association with the first valve andsecond valve to control the mixing of the first subsidiary stream andthe second subsidiary stream. The resulting partially cooled stream ofeither the first subsidiary stream or the second subsidiary stream or amixture of both the first and second subsidiary streams is directed tothe turboexpander. The exhaust stream from the turboexpander is thendirected to the lower pressure column in the two-column or three columndistillation column system. The cooling or supplemental refrigerationcreated by the expansion of the exhaust stream is thus imparted directlyto the lower pressure column thereby alleviating some of the coolingduty of the main heat exchanger.

In all contemplated embodiments, the aforementioned components of thefeed air streams, namely oxygen, nitrogen, and argon are separatedwithin the distillation column system 70 that includes a higher pressurecolumn 72 and a lower pressure column 74 using a well-known process offractional distillation. It is understood that if argon were a necessaryproduct, an argon column 76 could be incorporated into the distillationcolumn system 70. In the fractional distillation of air, the higherpressure column 72 typically operates at a pressure in the range frombetween about 20 bar(a) and about 60 bar(a) whereas the lower pressurecolumn 74 typically operates at pressures between about 1.1 bar(a) andabout 1.5 bar(a).

The higher pressure column 72 and the lower pressure column 74 arepreferably linked in a heat transfer relationship such that anitrogen-rich vapor column overhead, extracted from the top of higherpressure column 72 as a stream 73, is condensed within acondenser-reboiler 75 located in the base of lower pressure column 74against boiling an oxygen-rich liquid column bottoms 77. The boiling ofoxygen-rich liquid column bottoms 77 initiates the formation of anascending vapor phase within lower pressure column 74. The condensatefrom the condenser-reboiler 75 is a liquid nitrogen containing stream 81that is that divided into streams 83 and 84 and directed as refluxstreams to the higher pressure column 72 and the lower pressure column74, respectively to initiate the formation of descending liquid phasesin such columns.

In some of the illustrated embodiments, exhaust stream 64 is introducedinto the higher pressure column 72 along with the cooled, compressedboiler air stream 46A and preferably a third subsidiary stream 44. Insuch embodiments, the third subsidiary stream 44 is a medium pressureair stream at a pressure of between about 20 bar(a) and about 60 bar(a)that is discharged from the main heat exchanger 52 at the cold-endtemperature and bypasses the turboexpander 62. This stream 44 would savepower by liquefying air at a lower pressure than the boiler air 46, andprevent any dead pass in the heat exchanger 50 during differentoperation modes with a different nozzle.

Within the higher pressure column 72, there is a mass transfer occurringbetween an ascending vapor phase with a descending liquid phase that isinitiated by reflux stream 83 to produce a crude liquid oxygen columnbottoms 86, also known as kettle liquid and the nitrogen-rich columnoverhead 87. A plurality of mass transfer contacting elements,illustrated as trays 71 are used to facilitate the mass transfer betweenthe ascending vapor and descending liquid in the higher pressure column72. A stream 88 of the crude liquid oxygen column bottoms 86 or kettleliquid is withdrawn from the higher pressure column 72, subcooled insubcooling units 99A and 99B and expanded in an expansion valve 96 tothe pressure at or near that of the lower pressure column 74 and is thenintroduced into the lower pressure column 74 for further distillation.

Lower pressure column 74 is also provided with a plurality of masstransfer contacting elements, illustrated as structured packing elements79. As stated previously, the distillation process produces anoxygen-rich liquid column bottoms 77 extracted as an oxygen-rich liquidstream 90 and a nitrogen-rich vapor column overhead 91 that is extractedas a nitrogen product stream 95. The oxygen-rich liquid stream 90 ispreferably pumped via pump 180 to a higher pressure and then taken as ahigh pressure liquid oxygen product stream 185 and/or directed to themain heat exchanger where it is vaporized to produce a high pressureoxygen vapor product stream 190.

Additionally, a waste nitrogen stream 93 is also extracted from thelower pressure column 74 to control the purity of nitrogen productstream 86. Both the nitrogen product stream 95 and nitrogen waste stream93 are passed through subcooling units 99A and 99B designed to subcoolthe kettle liquid stream 88 to be used as reflux to the lower pressurecolumn 74. A portion of the subcooled kettle liquid stream mayoptionally be taken as a liquid product stream 98 and the remainingportion (shown as stream 94) may be introduced into lower pressurecolumn 74 after passing through expansion valve 96. After partialwarming by passage through subcooling units 99A and 99B, the nitrogenproduct stream 95 and nitrogen waste stream 93 are fully warmed withinmain heat exchanger 52 to produce a warmed nitrogen product stream 195and a warmed nitrogen waste stream 193. Although not shown, the nitrogenwaste streams may be used to regenerate the adsorbents within thepre-purification unit 28.

HEAT EXCHANGER DISTRIBUTOR

Conventional techniques to withdraw only a portion of a stream passingthrough passage of a brazed aluminum heat exchanger often involve acomplete withdrawal of the stream from the passage, and diverting aportion of the withdrawn stream for its desired use and subsequentreintroduction of the remaining portion of the stream back to the brazedaluminum heat exchanger. Such conventional partial extractions sufferfrom two disadvantages, namely a relatively high pressure dropattributable to the flow circuit outside the heat exchanger passage andan increased passage length of the heat exchanger required orattributable to the inactive length of the heat exchanger passagebetween the extraction point and the reintroduction point. Anotherproblem often encountered with existing schemes for partial withdrawalof a stream from a heat exchanger is that mal-distribution orinefficient use of heat transfer area could result, particularly whenusing the heat exchanger in off-design conditions.

To overcome these disadvantages, the embodiments of the main heatexchanger 52 in the present cryogenic air separation unit 10 preferablyinclude a distributor 200 as shown in FIG. 5 that is configured to allowpartial or complete withdrawal of a stream at a desired intermediatelocation along the length of the main heat exchanger 51. Advantageously,the illustrated distributor 200 avoids the full withdrawal of the streamand subsequent reintroduction of a portion of the stream back to theheat exchanger when only a portion of the stream needs to be extracted.The distributor design has little to no impact on flow distribution andalmost negligible impact on pressure drop compared to conventionalschemes.

The illustrated distributor 200 is a machined distributor manufacturedby using a solid aluminum slab 202 as shown in FIG. 5. The thickness ofthe aluminum slab 202 is preferably selected to match the height of thebrazed aluminum heat exchanger passage. The machined aluminum slab 202includes a plurality of vertically oriented channels 205 on one side ofthe aluminum slab 202 to permit the continuing stream to pass throughthe distributor 200. On the other side of the aluminum slab 202 is aplurality of slots 210. Each of the slots 210 has a first opening 212disposed at a top surface 214 of the aluminum slab 202 adjacent to thevertically oriented channels 205 and a second opening 216 disposed at anend edge 218 of the aluminum slab 202. Each of the slots 210 furtherinclude a first angled path section 220 extending from the first opening212 and a second horizontally oriented path section 222 extending fromthe first angled path section 220 to the second opening 216. Theplurality of slots 210 are configured to receive a portion of the streamat the first opening 212 and divert said portion via the first angledpath section 220 and the second horizontally oriented path section 222to the second opening 216. The portion of the stream then exits thealuminum slab 202 via the second opening 216 and exits the brazedaluminum heat exchanger 52 via a nozzle in an orientation generallyorthogonal to the flow path orientation of the heat exchanger passage.

A similar distributor can also be used to inject a stream into thebrazed aluminum heat exchanger via the plurality of slots by receiving astream at the second opening and passing the received stream via thesecond horizontally oriented path section to the angled or inclinedfirst path section and through the first opening where it joins thecontinuing stream within the heat exchanger.

The dimensions of the vertical oriented channels 205 on the one side ofthe aluminum slab 202 and the slots 210 on the other side of thealuminum slab 202 are selected based on the expected flow rates of thestreams to achieve the desired splits of flows between the withdrawn (orinjected) portion of the stream and the remaining carry-on portion ofthe stream. Unlike some of the conventional heat exchanger distributors,the illustrated distributor design maintains flow distribution withinthe heat exchanger passages and full utilization of heat transfer areafor off-design conditions.

OPERATING MODES FOR LIQUID PRODUCT MAKE

Referring again back to FIG. 1, some embodiments of the presentcryogenic air separation unit are configured for operating in either alow liquid make first mode with the turboexpander configured to expandonly the first subsidiary stream or a high liquid make second mode withthe turboexpander configured to expand only the second subsidiarystream. The low liquid make first mode is targeted to produce liquidproducts in the amount of about 2.5 percent or less of the total feedair stream. Conversely, the high liquid make second mode is targeted toproduce liquid products in the amount of about 4.5 percent or more ofthe total feed air stream.

The high liquid make second mode may be desired during the later yearsof the cryogenic air separation unit when the merchant liquid market hasdeveloped sufficiently while the low liquid make first mode may bepreferred during the initial few years (i.e. liquid ramp years) aftercommissioning of the cryogenic air separation unit when the merchantliquid market is not yet matured. Of course, there could be scenarioswhere it is just the opposite is true and the high liquid make secondmode is desired during early years or early phases of a project whilethe low liquid make first mode may be desired several years afterstart-up.

Alternatively, more rapid changes in product slate or liquid productrequirements may occur as a result of customer product demand issueseither for more liquid products or more pressurized gaseous productsoccurs. In such situations, the present cryogenic air separation unit iscapable of switching between the high liquid make first mode and the lowliquid make second mode. Still further, an operator can also switchbetween the low liquid make second mode and the high liquid make firstmode based on other operating characteristics such as cost of power, asthe power consumption is greater in the high liquid make first modecompared to the low liquid make second mode.

When integrated with a cryogenic air separation unit that includesturbine air stream bypass configuration, as shown in FIG. 3 thecryogenic air separation unit may operate in yet additional modes,including a third mode where the cryogenic air separation unit is in alow liquid make configuration with all or a portion of the turbine airstream bypassing at least one of the one or more turbine air streamcompressors prior to entering the main heat exchanger and theturboexpander configured to expand only the first subsidiary stream.Alternatively, the cryogenic air separation unit may operate in a highliquid make fourth mode with none of the turbine air stream bypassingany of the one or more turbine air stream compressors prior to enteringthe main heat exchanger in which case, the turboexpander is configuredto expand only the second subsidiary stream, which is equivalent to thefirst operating mode in the embodiment of FIG. 1. The other two modesare best characterized as medium liquid make modes, including a fifthmode with none of the turbine air stream bypasses any of the one or moreturbine air stream compressors prior to entering the main heat exchangerand the turboexpander configured to expand only the first subsidiarystream (which is similar to the second high liquid mode with referenceto FIG. 1) and a sixth mode where all or a portion of the turbine airstream bypasses at least one of the one or more turbine air streamcompressors prior to entering the main heat exchanger and theturboexpander configured to expand only the second subsidiary stream.

Some other embodiments of the described cryogenic air separation unitsare configured to operate to include a potential medium liquid makevariable mode where the turboexpander is configured to expand a mixtureof the first subsidiary stream and the second subsidiary stream. In suchembodiments, such as those illustrated in FIGS. 2 and 4, a third valveis disposed between the main heat exchanger and the turboexpander andconfigured to control the mixing of the first subsidiary stream and thesecond subsidiary stream. By mixing the first subsidiary stream and thesecond subsidiary stream, the inlet temperature of the combined streamsent to the turboexpander may be varied or controlled to a temperaturesomewhere between the first temperature and the second temperature. Themedium liquid make variable mode is preferably targeted to produceliquid products in the amount of between about 2 percent and about 5percent of the total feed air stream.

EXAMPLES

A number of computer simulations were run using cryogenic air separationunit operating models to characterize: (i) relative power consumption;(ii) liquid product make; and (iii); lower column turbine (LCT)efficiency when operating the present cryogenic air separation units asdescribed herein in the above-described operating modes. Details of thevarious operating modes are evidenced by the listed temperatures,pressures and flows of selected streams depicted in the Figs.

As seen in Table 1, Low Liquid Mode represents the low liquid make firstmode of the embodiments shown in FIG. 1 wherein High Liquid Moderepresents the high liquid make second mode of the embodiments shown inFIG. 1. The respective temperatures, pressures and flows of selectedstreams are depicted as well as the liquid product makes, relative powerconsumption; and; the lower column turbine (LCT) efficiencies.

TABLE 1 Low Liquid Mode High Liquid Mode Flow T T Stream (kcfh) (K) P(bar) Flow (K) P (bar) Stream 29 7631 284.3 10.15 7646 284.3 12.0 Stream46 2364 96.96 53.04 2225 97.46 53 Stream 35 5245 284.3 10.15 5386 301.724.8 Stream 44 127.6 96.96 9.7 148.6 97.46 24.5 Stream 66 — — — 5237160.1 24.6 Stream 68 5245 131.3 9.7 Stream 61 — — — Stream 64 5117 1135.55 5237 109.9 5.7 Liquid 1.7% 6.9% Make Power 25901 kW 30121 kW Con-sumption LCT  90%  85% Turbine Efficiency

As seen in Table 2, Low Liquid Mode represents the low liquid make firstmode of the embodiments shown in both FIG. 1 and FIG. 3 wherein HighLiquid Mode represents the high liquid make second mode of theembodiments shown in FIG. 3. The Medium Liquid Modes represent changesto the flows split between the boiler air stream and the turbine airstream (with and without the turbine air stream bypass. The respectivetemperatures, pressures and flows of selected streams are depicted aswell as the liquid product makes, relative power consumption; and; thelower column turbine (LCT) efficiencies.

TABLE 2 Mode Medium Medium Liquid Mode Liquid Mode Low Liquid (NoTurbine (With Turbine High Liquid Mode Bypass) Bypass) Mode Flow T PFlow T P Flow T P Flow T P Stream (kcfh) (K) (bar) (kcfh) (K) (bar)(kcfh) (K) (bar) (kcfh) (K) (bar) Stream 29 7631 284.3 10.2 7631 284.310.2 7631 284.3 12.0 7644 284.3 10.9 Stream 46 2364 96.9 53.0 2003 96.951.7 2388 97.6 53.0 2148 98.1 53.0 Stream 35 5245 284.3 10.2 5606 301.715.3 5221 284.3 12.0 5461 301.7 21.8 Stream 44 128 96.9 9.7 128 96.915.3 213 96.4 11.7 302 98.1 51.5 Stream 66 — — — 5479 135.0 15.3 5008142.0 11.8 5461 155.6 21.6 Stream 68 5245 131.3 9.7 — — — — — — — — —Stream 61 — — — — — — — — — Stream 64 5117 113 5.6 5479 102.7 5.6 5008116.3 5.6 5159 113 5.9 Liquid 1.7% 3.3% 3.0% 5.6% Make Power 25901 kW27496 kW 26645 kW 29566 kW LCT  90%  88%  90%  85% Efficiency

Although the present invention has been discussed with reference to oneor more preferred embodiments, as would occur to those skilled in theart that numerous changes and omissions can be made without departingfrom the spirit and scope of the present inventions as set forth in theappended claims.

What is claimed is:
 1. An air separation unit having one or moredistillation columns for the fractional distillation of a feed airstream and configured to produce one or more liquid products, the airseparation unit comprising: a main compressor arrangement configured tocompress the feed air stream; an adsorption based pre-purifierconfigured for removing water vapor, carbon dioxide, nitrous oxide,hydrocarbons or other contaminants from the compressed feed air streamto produce a compressed and purified feed air stream; a feed air streamcircuit configured to divide the compressed and purified feed air streaminto at least two streams including a boiler air stream and a turbineair stream; one or more boiler air stream compressors configured tocompress the boiler air stream; a main heat exchanger configured toreceive the compressed boiler air stream and cool the compressed boilerair stream to a cold-end temperature via indirect heat exchange with anoxygen stream from at least one distillation column of the airseparation unit; one or more turbine air stream compressors configuredto compress all or a portion of the turbine air stream; the main heatexchanger further configured to receive the turbine air stream anddischarge a first subsidiary stream at a first temperature warmer thanthe cold-end temperature and a second subsidiary stream at a secondtemperature colder than the first temperature but warmer than thecold-end temperature; a turboexpander configured to expand the firstsubsidiary stream or the second subsidiary stream to produce an exhauststream having an outlet temperature and wherein the exhaust stream isintroduced into the at least one distillation column of the airseparation unit; a first valve disposed between the main heat exchangerand the turboexpander and configured to adjust the flow of the firstsubsidiary stream and therefore, an inlet temperature to theturboexpander and the outlet temperature; a second valve disposedbetween the main heat exchanger and the turboexpander and configured toadjust the flow of the second subsidiary stream and therefore, the inlettemperature to the turboexpander and the outlet temperature; and the oneor more distillation columns configured for fractional distillation ofthe streams into a plurality of products including the one or moreliquid products; wherein the air separation unit is further configuredto operate in a low liquid make mode with the turboexpander configuredto expand only the first subsidiary stream: wherein the air separationunit is configured to operate in a high liquid make mode with theturboexpander configured to expand only the second subsidiary stream. 2.The air separation unit of claim 1, wherein the low liquid make modeproduces the liquid products in the amount of about 2.5 percent or lessof the total feed air stream.
 3. The air separation unit of claim 1,wherein the high liquid make mode produces the liquid products in theamount of about 4.5 percent or more of the total feed air stream.
 4. Theair separation unit of claim 1, wherein the air separation unit is stillfurther configured to operate in a medium liquid make variable mode withthe turboexpander configured to expand a mixture of the first subsidiarystream and the second subsidiary stream.
 5. The air separation unit ofclaim 4, further comprising a third valve disposed between the main heatexchanger and the turboexpander and configured to adjust the mixing ofthe first subsidiary stream and the second subsidiary stream andtherefore, the inlet temperature to the turboexpander and the outlettemperature.
 6. The air separation unit of claim 4, wherein the mediumliquid make variable mode produces the liquid products in the amount ofbetween about 2 percent and about 8 percent of the total feed airstream.
 7. The air separation unit of claim 1, wherein the firsttemperature is between about 140 Kelvin and about 220 Kelvin.
 8. The airseparation unit of claim 1, wherein the second temperature is betweenabout 130 Kelvin and about 140 Kelvin.
 9. The air separation unit ofclaim 1, wherein the one or more boiler air stream compressors areconfigured to compress the boiler air stream to a pressure between about25 bar(a) and about 70 bar(a).
 10. The air separation unit of claim 1,wherein the turbine air stream compressors are configured to compressthe turbine air stream to a pressure between about 20 bar(a) and about60 bar(a).
 11. The air separation unit of claim 1, wherein the main heatexchanger is further configured to discharge a third subsidiary streamat the cold-end temperature and at a medium pressure greater than thepressure of the compressed and purified feed air stream and less thanthe pressure of the compressed boiler air stream, wherein the thirdsubsidiary stream bypasses the turboexpander and is directed to the atleast one distillation column of the air separation unit.
 12. The airseparation unit of claim 1, wherein the feed air stream circuit isfurther configured to divide the compressed and purified feed air streamsuch that the turbine air stream is between about 60 percent and 75percent of the compressed and purified feed air stream.
 13. The airseparation unit of claim 1, further comprising a first distributordisposed within the main heat exchanger and wherein the first valve isan on-off valve in fluid communication with the first distributor andconfigured to be in an open position allowing flow of the firstsubsidiary stream to exit the main heat exchanger or a closed positionpreventing flow of the first subsidiary stream from exiting the mainheat exchanger.
 14. The air separation unit of claim 13, furthercomprising a second distributor disposed within the main heat exchangerand wherein the second valve is an on-off valve in fluid communicationwith the second distributor and configured to be in an open positionallowing flow of the second subsidiary stream to exit the main heatexchanger when the first valve is in the closed position or in a closedposition preventing flow of the first subsidiary stream from exiting themain heat exchanger when the first valve is in the open position. 15.The air separation unit of claim 1, wherein the one or more turbine airstream compressors further comprise at least one turbine loaded boostercompressor.
 16. An air separation unit having one or more distillationcolumns for the fractional distillation of a feed air stream andconfigured to produce one or more liquid products, the air separationunit comprising: a main compressor arrangement configured to compressthe feed air stream; an adsorption based pre-purifier configured forremoving water vapor, carbon dioxide, nitrous oxide, hydrocarbons orother contaminants from the compressed feed air stream to produce acompressed and purified feed air stream; a feed air stream circuitconfigured to divide the compressed and purified feed air stream into atleast two streams including a boiler air stream and a turbine airstream; one or more boiler air stream compressors configured to compressthe boiler air stream; a main heat exchanger configured to receive thecompressed boiler air stream and cool the compressed boiler air streamto a cold-end temperature via indirect heat exchange with an oxygenstream from at least one distillation column of the air separation unit;one or more turbine air stream compressors configured to compress theturbine air stream; a bypass circuit configured to direct all or aportion of the turbine air stream to bypass at least one of the one ormore of compressors; the main heat exchanger further configured toreceive the turbine air stream and discharge a first subsidiary streamat a first temperature warmer than the cold-end temperature and a secondsubsidiary stream at a second temperature colder than the firsttemperature but warmer than the cold-end temperature; a turboexpanderconfigured to expand the first subsidiary stream or the secondsubsidiary stream to produce an exhaust stream having an outlettemperature and wherein the exhaust stream is introduced into the atleast one distillation column of the air separation unit; a first valvedisposed between the main heat exchanger and the turboexpander andconfigured to adjust the flow of the first subsidiary stream andtherefore, an inlet temperature to the turboexpander and the outlettemperature; a second valve disposed between the main heat exchanger andthe turboexpander and configured to adjust the flow of the secondsubsidiary stream and therefore, the inlet temperature to theturboexpander and the outlet temperature; and the one or moredistillation columns configured for fractional distillation of thestreams into a plurality of products including the one or more liquidproducts; wherein the air separation unit is configured to operate in alow liquid make mode with all or a portion of the turbine air streambypassing at least one of the one or more compressors prior to enteringthe main heat exchanger and the turboexpander configured to expand onlythe first subsidiary stream; wherein the air separation unit isconfigured to operate in a high liquid make mode with none of theturbine air stream bypassing any of the one or more compressors prior toentering the main heat exchanger and the turboexpander configured toexpand only the second subsidiary stream; wherein the air separationunit is configured to operate in a medium liquid make mode with none ofthe turbine air stream bypassing any of the one or more compressorsprior to entering the main heat exchanger and the turboexpanderconfigured to expand only the first subsidiary stream; and wherein theair separation unit is configured to operate in an alternate mediumliquid make mode with all or a portion of the turbine air streambypassing at least one of the one or more compressors prior to enteringthe main heat exchanger and the turboexpander configured to expand onlythe second subsidiary stream.
 17. The air separation unit of claim 16,wherein the high liquid make mode produces the liquid products in theamount of about 4.5 percent or more of the total teed air stream. 18.The air separation unit of claim 16, wherein the low liquid make modeproduces the liquid products in the amount of about 2.5 percent or lessof the total feed air stream.
 19. The air separation unit of claim 16,wherein the medium liquid make mode produces the liquid products in theamount of between about 2 percent and about 5 percent of the total feedair stream.
 20. The air separation unit of claim 16, wherein thealternate medium liquid make mode produces the liquid products in theamount of between about 2 percent and about 5 percent of the total feedair stream.
 21. The air separation unit of claim 16, further comprisinga third valve disposed between the main heat exchanger and theturboexpander and configured to adjust the mixing of the firstsubsidiary stream and the second subsidiary stream and therefore, theinlet temperature to the turboexpander and the outlet temperature. 22.The air separation unit of claim 20, wherein the air separation unit isstill further configured to operate in a liquid make variable mode withnone of the turbine air stream bypassing any of the one or morecompressors prior to entering the main heat exchanger and theturboexpander configured to expand a mixture of the first subsidiarystream and the second subsidiary stream.
 23. The air separation unit ofclaim 22, wherein the liquid make variable mode produces the liquidproducts in the amount of between about 2 percent and about 8 percent ofthe total feed air stream.
 24. The air separation unit of claim 21,wherein the air separation unit is still further configured to operatein a liquid make variable mode with all or a portion of the turbine airstream bypassing at least one of the one or more compressors prior toentering the main heat exchanger and the turboexpander configured toexpand a mixture of the first subsidiary stream and the secondsubsidiary stream.
 25. The air separation unit of claim 24, wherein theliquid make variable mode produces the liquid products in the amount ofbetween about 2 percent and about 5 percent of the total feed airstream.
 26. The air separation unit of claim 16, wherein the firsttemperature is between about 140 Kelvin and about 220 Kelvin.
 27. Theair separation unit of claim 16, wherein the second temperature isbetween about 130 Kelvin and about 140 Kelvin.
 28. The air separationunit of claim 16, wherein the one or more boiler air stream compressorsare configured to compress the boiler air stream to a pressure betweenabout 25 bar(a) and about 70 bar(a).
 29. The air separation unit ofclaim 16, wherein the turbine loaded booster compressor is configured tocompress the turbine air stream to a pressure between about 20 bar(a)and about 60 bar(a).
 30. The air separation unit of claim 16, whereinthe main heat exchanger is further configured to discharge a thirdsubsidiary stream at the cold-end temperature and at a medium pressuregreater than the pressure of the compressed and purified feed air streamand less than the pressure of the compressed boiler air stream, whereinthe third subsidiary stream bypasses the turboexpander and is directedto the at least one distillation column of the air separation unit. 31.The air separation unit of claim 16 wherein the feed air stream circuitis further configured to divide the compressed and purified feed airstream such that the turbine air stream is between about 60 percent and75 percent of the compressed and purified teed air stream.
 32. The airseparation unit of claim 16, further comprising a first distributordisposed within the main heat exchanger and wherein the first valve isan on-off valve in fluid communication with the first distributor andconfigured to be in an open position allowing flow of the firstsubsidiary stream to exit the main heat exchanger or a closed positionpreventing flow of the first subsidiary stream from exiting the mainheat exchanger.
 33. The air separation unit of claim 32, furthercomprising a second distributor disposed within the main heat exchangerand wherein the second valve is an on-off valve in fluid communicationwith the second distributor and configured to be in an open positionallowing flow of the second subsidiary stream to exit the main heatexchanger when the first valve is in the closed position or in a closedposition preventing flow of the first subsidiary stream from exiting themain heat exchanger when the first valve is in the open position. 34.The air separation unit of claim 16, wherein the one or more turbine airstream compressor further comprise at least one turbine loaded boostercompressor.